Metamorphic Rocks

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Metamorphic Rocks
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Metamorphic Rocks

• Chemical and physical alterations of pre-existing
  rocks brought about by heat and pressure
• 2 Types
• Regional metamorphism
• Contact Metamorphism

• 4 Main Factors
• Temperature
• Fluids
• Pressure
• Deviatoric (directional) Stress

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Regional Metamorphism

• Large scale deformation and alteration brought
  about by mountain building (orogeny) and related
  processes
• Usually on a larger scale

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Contact Metamorphism

• Smaller scale alteration adjacent to intruding
  magma

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Changes in Temperature and Pressure

• The heat source is Earths internal heat.
• Burial is inevitably accompanied by an increase
  in pressure due to the weight of the overlying
  rocks.

Both temperature and pressure increase with
depth in the Earth resulting in higher
metamorphic grades with depth.
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Once we have melting, it is no longer a
metamorphic rock, it is igneous!!
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Differential Stress

• Differential stress occurs if pressure is
  different in different directions.
• Sometimes called Deviatoric stress

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Differential Stress

• Foliation vs Lineation

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Foliation
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Foliation--Slaty Cleavage

• Alters orientation of existing minerals

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Foliation--Schistocity

• Alters orientation of existing minerals
• Crystallization of new minerals

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Foliation--Schistocity
Picture of schist

• Parent rock is often a fine grained rock
• Shale, mudstone, siltstone

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Foliation--Gneissic banding

• Alters orientation of existing minerals
• Crystallization of new minerals
• Minerals become segregated into bands

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Foliation--Gneissic banding

• Parent rock is often a fine grained rock (as is
  both slate and schist)
• Shale, mudstone, siltstone

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Sedimentary rock with distinct bedding.
Original Rock
Metamorphism
Schist
Slate
Gneiss
Foliation perpendicular to direction of maximum
differential stress
Foliation and crystallization perpendicular to
direction of maximum differential stress
Foliation, crystallization and differentiation
perpendicular to direction of maximum
differential stress
http//www.tulane.edu/sanelson/geol212/contactmet
a.htm
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Index Mineral a mineral in a metamorphic rock
which indicates a certain metamorphic grade
Isograd line marking the first appearance of an
index mineral on a map or cross-section
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Metamorphic Grade